In process automation technology, a large number of different sensors are applied for registering process variables. Examples of such sensors include pH sensors, gas sensors, flow sensors, mass flow sensors and the like. In modern industrial plants, such sensors are often operated under difficult environmental conditions. For example, the sensors are exposed to corrosive chemicals, heat, vibration and the like.
The sensors are arranged, in such case, at measuring points composed of a sensor element and a sensor cable, which connects the sensor with a measurement transmitter. In some cases of application, the sensor is placed in a retractable assembly of stainless steel or a durable synthetic material, such as a plastic, in order to enable insertion of the sensor into, and withdrawal from, the process.
Known from German patent, DE 10 2007 048 812 A1 is a cable circuit with digital signal conditioning. The cable circuit serves for connecting a sensor module with a measurement transmitter. The cable circuit includes a contactless interface for signal transmission between the circuit formed in the cable and the sensor module, wherein the sensor module is galvanically isolated from the cable circuit and signal transmission between the cable circuit and the sensor module occurs by optical, inductive or capacitive means. Furthermore, also the energy supply for the sensor module occurs via this wireless interface. The wireless interface is, in such case, positioned partly in the cable circuit and partly in the sensor module, wherein the two parts of the wireless interface are arranged lying opposite one another by introducing the cable circuit into the sensor module. The housings of the sensor module and the cable have a relatively small diameter, for example 12 mm. The sensor can be applied in a retractable assembly (see below), so that the maximum outer dimensions are predetermined. As already mentioned, a part of the signal conditioning occurs in a circuit in the sensor and/or in the cable. The components required for conditioning, such as a microprocessor, other active components as well as passive components, must be accommodated in the predetermined spatial conditions. The wall thickness of the housing of the cable, respectively of the sensor module, as caused by the relatively small housing diameters, is very small and lies at, for instance, 1-2 mm.
Especially when the wireless interface is embodied as a stray field transformer, whose primary winding is arranged in the cable circuit and whose secondary winding is arranged in the sensor module, there arise in the energy- and data transmission magnetic stray fields, which lie also outside of cable and sensor housings. Through the installation of such a plug connecting unit in a, for example, tubular, stainless steel assembly for inserting the sensor module into a medium to be examined, the stainless steel assembly influences the stray fields. On the one hand, the stainless steel assembly acts like a second secondary winding, which is electrically short circuited. On the other hand, there arise in the stainless steel assembly eddy current losses. Therewith, the effective inductance and the effective quality of the primary winding are changed in such a manner that the working point of the primary winding is changed and the efficiency of cable including sensor is reduced. This has the result that both the energy—as well as also the data transmission are negatively influenced.